This invention relates to the field of information transmission using optical fibers, and more particularly to the design of an optical transmission system that employs stimulated Raman scattering for amplification.
The growth in optical communications has been fueled by the extraordinary bandwidth that is available on optical fiber. Such bandwidth enables thousands of telephone conversations and television channels to be transmitted simultaneously over a hair-thin fiber that is made from a high-quality glass material. Nevertheless, similar to electrical signals, optical signals experience loss during transmission and must be periodically amplified, although the need for amplification is reduced by increasing the power of the optical signals to be transmitted. To handle increased optical power, fibers having larger effective areas have been developed in order to avoid the nonlinear effects associated with high power density.
Optical amplification is more cost effective than the conversion of optical signals into electrical signals, which are amplified and then converted back into optical signals. One amplification technique involves: doping a length of optical fiber with rare earth materials such as erbium or praseodymium; pumping optical energy into the length of optical fiber at a wavelength that is different than the wavelength of the optical signal to be amplified; and propagating the optical signal along the length of rare-earth-doped fiber to extract energy at its own wavelength. Erbium-doped fiber is used to amplify optical signals having wavelengths in the 1550 nanometer (nm) region where there is a transition in the Er3+ dopant ion, whereas praseodymium is useful in the 1310 nm region. Although such amplifiers represent a significant improvement over the above-described electronic amplification method, the price of such optical amplifiers is still highxe2x80x94e.g., $25,000 to $50,000 each. In addition, erbium amplifiers have to be driven by one or two laser-diode pumps; and, if a pump quits, the whole system goes down. (The erbium is not transparently turned off, but it uses a prelevel laser that absorbs the signal when it goes off.) Praseodymium amplifiers have some of the same problems and, in addition, are made of a fluoride-base fiber that is brittle and fragile.
Another optical amplification technique takes advantage of a phenomenon known as stimulated Raman scattering (SRS), which has substantial benefits including: low costxe2x80x94e.g., $3000 to $4000 each; operation at all wavelengths; and use of the transmission fiber itself for amplification. Indeed, this technique relies on an intrinsic property of the material of the fiber and does not require the presence of any special dopant in the fiber such as erbium. Accordingly, it is frequently desirable to use Raman amplification in optical transmission systems.
Raman amplification involves the introduction of an optical pump signal onto the transmission fiber, and for a given pump power Raman amplification efficiency increases as optical power density increases. However, if the power density of the fiber becomes too great, then optical transmission signals experience undesirable nonlinear effects. Accordingly, it is desirable to reconcile the need for low power density, which reduces nonlinear effects, with the need for high power density, which increases Raman amplifier efficiency, in an optical transmission system.
In accordance with the present invention, a Raman-amplified optical transmission system includes a source of optical transmission signals that is connected to one end of a first optical fiber having an effective area. The other end of this fiber is connected to a second optical fiber having a substantially smaller effective area. Optical pump signals are coupled to the second optical fiber that cause it to exhibit stimulated Raman scattering and, hence, amplification of the optical transmission signals.
In an illustrative embodiment of the present invention, the optical pump signals propagate along the second optical fiber in a direction that is opposite the direction of the optical transmission signals.
Various cable configurations are useful in connection with the present invention that preferably include an equal number of large-effective-area fibers, i.e., Aeffxe2x89xa770 xcexcm2, and small-effective-area fibers, i.e., Aeffxe2x89xa660 xcexcm2 within the same cable. One of the cable configurations includes a planar array of optical fibers that are bound together in a matrix material; whereas in another configuration, groups of optical fibers are enclosed within one or more plastic tubes.